Large pore synthetic polymer membranes

Abstract

Highly asymmetric polymeric membranes with large pores which yield bubble points in the range of 0.5 to 25 psid and superior flow characteristics. The membranes can be cast from both metastable dispersions and from homogenous casting formulations. The technique of synthesis involves exposure of the cast membrane to humid air to create large surface pores on the exposed side.

What is claimed is: 
     
       1. A method for preparing a polymer membrane having a first and second surface and having a relatively large pore size in the first surface, a substantially asymmetric structure, and an enhanced flow rate, comprising: preparing a homogeneous casting solution comprising a polymer, a solvent for the polymer, and a non-solvent for the polymer at a selected casting temperature;   casting the dispersion into a thin layer at the casting temperature;   contacting the cast layer with a pore forming atmosphere for a period of time sufficient to form surface pores greater than 1.2 microns; and   quenching the cast layer with a non-solvent quench liquid in which the solvent is miscible and in which the polymer is substantially insoluble to precipitate the polymer as an integral membrane;   recovering the membrane from the quench liquid, wherein the membrane has a region of substantial asymmetry through at least fifty percent of the membrane, and wherein the membrane further has a substantially isotropic region extending from the first surface to the region of substantial asymmetry.   
     
     
       2. The method of claim 1, wherein the membrane recovered in the recovering step possesses a substantially isotropic structure extending from the first surface to a point between the first surface and the second surface and through at least twenty percent of the membrane. 
     
     
       3. An integral asymmetric polysulfone membrane having a first and second surface, having a first surface pore mean diameter greater than 1.2 microns and having a substantially asymmetric structure through at least fifty percent of the membrane, prepared by the method of claim 1. 
     
     
       4. In a process to prepare an integral highly asymmetric polymer membrane having a first and second surface, the improvement comprising: contacting the cast layer with a gaseous atmosphere with a pore forming atmosphere for a period of time sufficient to form surface pores greater than 1.2 microns in the first surface, a substantially asymmetric structure extending through at least fifty percent of the membrane, and a substantially isotropic structure extending from the first surface to the substantially asymmetric structure.   
     
     
       5. The process of claim 4, wherein the cast layer is contacted with the pore forming atmosphere for a pericd of time sufficient to form a substantially isotropic structure extending from the first surface to a point between the first surface and the second surface and through at least twenty percent of the membrane.